I set up a form so that participants can sign up for conference sessions. I need to limit participation to 15 people per session. How can I automaticcally make that session NOT available once 15 sign up?

You have to manually close the form yourself. There is no way to automate that process. You can turn on Email Notifications (under the Options tab). This way you can receive an email every time someone submit. Note that email notifications is a paid feature (you need a paid subscription).